1. Field of the Invention
This invention relates to an ophthalmologic apparatus, and more particularly to an ophthalmologic apparatus having a projection optical system for projecting a laser beam onto a prescribed point of an eye under examination as a spot image.
2. Description of the Prior Art
Ophthalmologic apparatuses of this type include, for example, the laser beam coagulation apparatus which is used for directing a laser beam onto living tissue within the eye for raising the tissue to a high temperature and causing it to thermocoagulate. In using this apparatus, it is necessary to adjust the diameter of the laser beam spot according to the size of the portion to be subject to thermocoagulation. Conventionally, a zoom lens optical system has been used for this adjustment of the spot diameter.
The zoom lens optical system used in such an ophthalmologic apparatus comprises first and second lens groups which can be moved with respect to each other to provide variable power capability. Since the lens groups are moved so that the surface from which the laser beam is emitted and the point at which the spot image is formed fall at the conjugate points of the zoom lens optical system, it is consistently possible to obtain a spot image that is in good focus and has a sharp edge.
In the case of conducting laser beam coagulation, however, it is not always desirable to use a sharp-edged image and there are cases in which it is better to conduct the optical coagulation with a spot image having a fuzzy edge. For this, it is necessary to form the spot image at a position that is not at the focal point so as to separate it from the conjugate position. Where a zoom lens is used to realize variable power capability, however, the aforesaid movement of the lens groups is controlled so as to maintain the conjugate relationship. Thus the spot image is always in focus and it is not possible to obtain a spot image with a fuzzy edge.